1. Field of the Invention
This invention relates generally to earthboring tools for boring a hole.
2. Description of the Related Art
Earthboring tools are commonly used to bore holes by cutting through earthen annulus. Such holes may be bored for many different reasons, such as drilling for oil, minerals and water. One type of earthboring tool used for boring is a rotary earth bit. Several examples of rotary earth bits are disclosed in U.S. Pat. Nos. 3,550,972, 3,847,235, 4,136,748, 4,427,307, 4,688,651, 4,741,471 and 6,513,607. A rotary earth bit generally includes an earth bit body comprised of three lugs which form a bit chamber. A cutting cone is rotatably mounted to each lug with a bearing journal. The bearing journal generally includes a bearing system having ball and roller bearings which engage the cutting cone. The lug rotates in response to the rotation of the earth bit. The cutting cones are engaged with the roller and ball bearings and rotate, in response to contacting earthen annulus, about the bearing journal.
A lubricating material is often used to lubricate the bearing system. The lubricating material is retained in a bearing cavity in fluid communication with the bearing system. The lubricating material is typically retained within the bearing cavity by using one or more sealing members, such as O-ring seals. It should be noted that the lubricating material is generally a fluid or grease, but it can include vapors thereof.
The lubricating material experiences changes in temperature during the operation of the earth bit, which causes corresponding changes in its pressure in the bearing cavity. A force is applied to the sealing member(s) in response to the pressure of the lubricating material in the bearing cavity. The force stresses the sealing member(s), which can cause them to fail. Seal failure will cause premature bearing failure leading to early bit failure, so it is desirable to decrease the amount of force applied to it.
Earth bits often include a grease passage in fluid communication with the bearing cavity and the bit chamber. Heat is generated from friction in the bearing system. The grease passage allows pressure generated by this heat to equalize between the bearing cavity and bit chamber. However, it is desirable to control the pressure of the lubricating material.